Hand tools having two pivotally connected handles for moving jaws that a user operates with one hand are well known in the art. The basic tool has three parts: a unitary first member having a first handle on one end and a first jaw on the other, a unitary second member having a second handle on one end and a second jaw on the other, and an axle pivotally connecting the two together. Scissors and sheet metal snips are examples of such hand tools operated by one hand.
In certain applications the mechanical advantage offered by the leverage of such a three part hand tool is insufficient making it difficult or impossible to achieve the desired result with one hand. For example, cutting a thick gauge of sheet metal with a simple sheet metal snip can require more hand pressure than the user can deliver, particularly when the cutting must be performed over a protracted period of time.
FIGS. 1 and 2 illustrate side elevation views of a prior art hand tool 500 shown in open and closed positions. The hand tool shown is generally referred to as an aviation snip. A second set of levers between the handles and the jaws compounds the force created by a person squeezing the handles when the force is transmitted to the jaws. This allows the user to cut thicker materials that would otherwise be difficult or impossible to cut with the strength of one hand. Hand tool 500 has a first jaw member 502 having a first end 504, an opposite second end 506, and an intermediate portion 508. Hand tool 500 also has a second jaw member 510 having a first end 512, an opposite second end 514, and an intermediate portion 516. Intermediate portion 516 of second jaw member 510 is pivotally connected to intermediate portion 508 of first jaw member 502 at a first pivot P1. Hand tool 500 also has a first handle member 518 having a first end 520, an opposite second end 522, and an intermediate portion 524. Intermediate portion 524 of first handle member 518 is pivotally connected to second end 514 of second jaw member 510 at a second pivot P2. Hand tool 500 also has a second handle member 526 having a first end 528, an opposite second end 530, and an intermediate portion 532. Intermediate portion 532 of second handle member 526 is pivotally connected to second end 506 of first jaw member 502 at a third pivot P3. First end 520 of first handle member 518 is pivotally connected to first end 528 of second handle member 526 at a seventh pivot P7. It is noted that pivot P7 is disposed between pivot P1 and pivots P2 and P3. Or put another way, when viewed as shown with pivot P1 the uppermost pivot, pivot P7 is below pivot P1 and above pivots P2 and P3. Hand tool 500 also includes a torsion spring 534 which biases first handle member 518 and second handle member 526 apart so that hand tool 500 resides in the open position of FIG. 1. A pivoting lock 536 cooperates with a shaft located at P2 to lock hand tool 500 in the closed position. The first jaw member 502 and second jaw member 510 are shaped and dimensioned so that they combine to form an aviation snip. They cross over each other as the hand tool is closed providing a cutting action such as in scissors. It is further noted that hand tool 500 is designed to be operated using only one hand.
While a simple metal shear may be satisfactory for certain applications where the material to be cut is relatively thin, and while a compound metal shear of the type shown as hand tool 500 above may be satisfactory for other applications where the material to be cut is somewhat thicker, it would be advantageous to have available yet another metal shear having even greater mechanical advantage for cutting material with one hand that could not be cut with either a simple or compound shear such as those shown in the prior art. Furthermore, such a metal shear having a greater mechanical advantage could also be used over a longer period of time by one hand to cut materials that could be cut by either of the other prior art devices for the short term.